Reactor control method

ABSTRACT

A method is provided for controlling the operating temperature of a catalytic reactor using a closed-loop system that provides for varying the reactor input and other operating parameters in order to maintain the operating temperature of the reactor at or near the initial setpoint temperature for operation of the reactor. In one example, maximum and minimum operating temperatures with a catalytic partial oxidation reactor are controlled, as well as maintaining control over the corresponding minimum required ratio of oxygen atoms to carbon atoms, such that the operating temperature within the reactor is maintained below the material limits but above threshold temperatures for coking.

CROSS-REFERENCE

This application is a divisional of U.S. patent application Ser. No.12/322,970, filed Feb. 9, 2009, which claims the benefit of U.S.Provisional Patent Application No. 61/063,952 filed Feb. 7, 2008. Theaforementioned U.S. patent applications in their entirety areincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a method for controlling the operatingtemperature of a catalytic reactor. While this is applicable todifferent types of catalytic reforming reactors, it is described herewith reference to a Catalytic Partial Oxidation Reactor since theconstraints of high temperature and coking are most acute. The presentinvention provides a method for protecting the catalyst from temperatureexcursions, minimizing coking, compensating for suboptimal mixing at thefeed, recovering from operation under coking regimes, and providing fora much longer run-time duration at maximum operating temperature, andperiodically or actively maintaining the minimum operating temperature.

BACKGROUND OF THE INVENTION Brief Description of the Related Art

There are many methods known in the art for controlling the operatingtemperature of a chemical reactor. Closed-loop systems are among theseknown methods. Typically, a controller is employed such that one or moreoutput variables of a system are tracked against a certain referencepoints over time. The controller varies the inputs to a system to obtainthe desired effect on the output of the system thereby maintaining theoutput variables at or near the reference points. Accordingly, aclosed-loop system for controlling the operating temperature of achemical reactor would monitor the reactant products or other operatingparameters such as operating temperatures, track the measurements andcompare such values to a desired reference. The system would provide forvarying the reactor input and other operating parameters in order tomaintain the operating temperature of the reactor at or near a referencepoint or reactor temperature setpoint.

Waterless catalytic partial oxidation (hereinafter referred to as“CPOx”) of liquid distillate fuels, such as, for example, diesel andJP8, with near complete conversion to Carbon (C₁) products is achallenging proposition. The general reaction is shown below:

C_(x)H_(y)+O₂ →mCO+nH₂+small amounts of CO₂ and H₂O

The practical ability to operate in this mode requires a reactor designthat provides high selectivity to the partial oxidation products CO andH₂ compared to the complete oxidation products CO₂ and H₂O. While somestudies have described CPOx of diesel, typically these have not beenoperated in “dry” mode (i.e., the reactions have been performed byadding some steam from an external source, or by partially burning someof the hydrocarbon feed to generate water in-situ, and/or by using anupstream process to remove the heavier ends from the fuel).

Moreover, CPOx of distillate fuel is made difficult due to carbonformation and/or excessively high reactor temperatures. In the presentinvention, in conjunction with reactor design, unique control algorithmspermit operational precision that addresses the constraints of dry CPOx.Thermodynamic considerations of dry CPOx, as well as the hurdles anddesign requirements identified to develop a dry CPOx reactor, also mustbe considered and explained here.

Due to the thermodynamic drive for coke formation, which can beexacerbated by inherent non-uniformities in the feed to a CPOx reactor,there is a need to diminish this coke formation and the subsequentcatalyst temperature spread resulting from increasing flow disparitiesthat arise. By limiting fuel flow over a certain span, the maximumoperating temperature of a reactor can serve as a measured processvariable and subsequently controlled in closed-loop fashion. However,under coking and/or suboptimal mixing/fuel-atomization/vaporizationconditions, having a maximum operating temperature within the reactor(hereinafter “Tmax”) set such that the minimum operating temperaturewithin the reactor (hereinafter, “Tmin”) is above the thermodynamicthreshold for coking will only be useful for a short time; inherent coldzones may develop, dropping the Tmin even though Tmax is held constant.In this case, attempting to increase Tmin at point in the reactor byadjusting the fuel input would have the undesirable effect of increasingTmax at another point in the reactor. Therefore, a periodic need for asecond closed-loop system controlling air or other means to increase theTmin is desirable.

It is an object of the present invention to provide a method forcontrolling the operating temperature of a reactor using a closed-loopsystem that provides for varying the reactor input and other operatingparameters in order to maintain the operating temperature of the reactorat or near the initial setpoint temperature for operation of thereactor. It is another object of the present invention to provide amethod for controlling the operating temperature of a reactor using aclosed-loop system that provides for controlling the operatingtemperature of a CPOx reactor which maintains temperatures within thecatalytic reactor below material limits but above threshold temperaturesfor coking.

DESCRIPTION OF THE INVENTION

A reforming reactor was tested for the dry reforming of distillatefuels. It was based on a small, modular catalytic reactor, whichemployed patented Microlith® substrate and catalyst technology availablefrom Precision Combustion, Inc. in North Haven, Conn. High conversion ofthe diesel feed was observed with high selectivity of the hydrogen inthe fuel converted to H₂. A control algorithm was developed for stablelong-term operation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a graphical depiction of coking and high temperatureregions associated with CPOx of liquid distillate fuels and anillustration of the desired operating region in accordance with thepresent invention.

FIGS. 2 and 3 provide a graphical depiction of reactor operatingtemperature trigger points for intervention by a closed-loop systemcontroller in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 below presents a graphical depiction (10) of the results ofthermodynamic calculations at ambient pressure defining the boundariesbetween the regions associated with reactor temperature and cokingconstraints. In these calculations, hydrocarbon distillate boiling rangefuels were represented by a surrogate blend of model compounds. In allcases, hydrocarbon conversion at equilibrium was complete.

CPOx operation occurs to the right of the line (12) defining theadiabatic temperature boundary. The coking region (14) is defined by thearea below the adiabatic temperature line (12) and its intersection withthe curve (16) defining the relationship between CPOx operatingtemperature and the corresponding minimum required ratio of oxygen atomsto carbon atoms (hereinafter referred to as the “O/C Ratio”) to avoidcarbon formation.

FIG. 1 indicates that higher temperatures ease the problem of carbonformation. The required feed O/C Ratio diminishes significantly as thepractical limit on operating temperature for catalytic processing isreached. The adiabatic temperatures associated with higher O/C Ratioscan substantially exceed the maximum acceptable limit for catalyticoperation and material durability. Therefore, FIG. 1 implies thatoperating at unduly high O/C Ratios to avoid carbon formation would leadto increasing reactor heat duties.

When operating in the coke-free region at or below the maximum operatingtemperature range, the higher the O/C Ratio, the lower the selectivityto H₂ and CO. Thus, it is desirable to operate at the lowest O/C Ratioconsistent with the avoidance of carbon formation while achievingacceptable CPOx product selectivities. As the temperature increases andthe required O/C Ratio declines, the adiabatic temperature rise likewisediminishes. Table 1 below illustrates some of the sensitivities ofcarbon formation to small variations in operating conditions.

TABLE 1 Calculated Dry CPOx Performance Variations Near the CarbonFormation Boundary Adiabatic Carbon Formation, O/C Ratio Tin, ° C.Temp., ° C. wt. % Hydrocarbon Feed Base Base Base 0.0 95% Base Base Base− 65 0.55 95% Base Base + 100 Base + 11 0.22 95% Base Base + 150 Base +52 0.14 95% Base Base + 200 Base + 94 0.09

A relatively small decrease in the O/C Ratio at a fixed inlettemperature results in the onset of carbon formation with a 65° C. loweradiabatic temperature. Progressive increases in the inlet temperature atthe decreased O/C Ratio level boost the adiabatic temperature back to,and beyond, the base level calculated; however, while progressivelyreducing the calculated equilibrium carbon level, the increases do notcompletely eliminate it even at adiabatic temperatures exceeding theexpected maximum allowable operating level. This indicates theimportance of maintaining the proper minimum O/C Ratio if even smallamounts of carbon formation cannot be tolerated. Thus, dry CPOx involvesa balance between avoiding coke formation by managing the O/C Ratiowhile ensuring that acceptable temperatures are maintained to avoidexceeding any material's limitations and/or a decline in productselectivity.

This balance is indicated by the Operating Window for Near AdiabaticCPOx (18) depicted in FIG. 1. These thermodynamic calculations provideuseful directional guidance. Actual experimental observations, however,will depend upon such things as coke formation kinetics and experimentalnon-idealities.

In one embodiment of the present invention, fuel was pressurized by afuel pump and metered through a fuel gauge. A fuel-air mixture waspassed to the catalytic reactor using a spray nozzle. Compressed air wassupplied to the system and was metered by mass flow controllers. Ambientair without external pre-heating was used. Water flow (when employed forother reforming reactions) was metered by a calibrated piston pump andwas passed through an electrically heated vaporizer prior to mixing withthe air stream. The fuel/air (and steam when applicable) mixture enteredthe catalyst bed where the reforming reaction occurred producing H₂ andCO. Peak reactor temperatures were maintained below the catalyst andsubstrate material limits. Reforming was performed using a catalyticreactor comprising the wire mesh-based substrate Microlith® coated witha selective Rh supported on alumina washcoat formulation. The decisionto augment dry reforming with some steam addition is dependent upon theinterplay between the desire to reduce the hydrocarbon feed rate and anyapplication-specific constraints. As noted above, the use of some steammay also be desirable for moderating any coke formation under reformingconditions.

One embodiment of the present invention for controlling the operatingtemperature of a reactor using a closed-loop system is graphicallydepicted in FIGS. 2 and 3. FIG. 2 illustrates reactor operatingtemperature measurements made at three locations in a reactor T1, T2,and T3, each at time t, designated by T1(t), T2(t), and T3(t),respectively. At each time t, the highest temperature is denoted asTmax(t), and similarly the minimum temperature at time t is Tmin(t). Inthe example depicted in FIG. 2, two points in time along the x-axis aredesignated as t=A and t=B as shown. At time t=A, the highest temperatureis T1(A), therefore Tmax(A) (i.e., Tmax at time A) is equal to T1(A). Attime B, however, T3 is the highest temperature, consequently Tmax(B) isequal to T3(B). In this manner, the function Tmax(t) and Tmin(t) iscalculated by the control system, as shown in FIG. 3, for use in thealgorithm described below.

An initial steady state temperature setpoint (hereinafter referred to as“Th”) is selected to deliver safe durable reactor operation. A shortterm allowable temperature setpoint (hereinafter referred to as“Th-max”) is selected to deliver safe operation with limited reactorlife. The controller adjusts reactor parameters in order to maintainTmax(t) at or near Th. Th may be held constant, as in the case of asimple temperature controller, wherein the reactor is held to a constantmaximum temperature having advantages well known in the art.

In the case of a CPOx reactor there are additional benefits associatedwith the closed-loop system control. Typically, the CPOx reactor isoperating at the edge of its material limits. Coking and small changesin fuel or air flows can cause large changes in temperature.Accordingly, the control system of the present invention is particularlyrelevant and beneficial to CPOx of liquid fuels

However, even if Tmax(t) is held constant, there may be conditions orproperties particular to the reactor which cause Tmin(t) to vary inundesirable ways; particularly to decrease making the resultanttemperature differential Tmax(t)−Tmin(t) to increase. The method of thepresent invention seeks to reduce the temperature differentialTmax(t)−Tmin(t) by temporarily increasing Th to Th-max, or byalternative control methods considered within the scope of the presentinvention which increase Tmax(t) while staying below Th-max.

One such alternative control method comprises increasing Th to Th-maxfor all times where temperature differential Tmax(t)−Tmin(t) exceeds aspecified threshold; or, in other words, increasing Th to Th-max untiltemperature differential Tmax(t)−Tmin(t) is less than a selectedthreshold. The controller seeks to maintain Tmax(t) equal to Th-max forthis duration. Another alternative method comprises increasing Th toTh-max until Tmin(t) is greater than a selected threshold.

The control method applied may take many forms to achieve the sameresult. Three non-exhaustive set of possible algorithms for a reactorwith feed comprising fuel and air only is presented below Suchalgorithms may be equally applied to a reactor which is also fed withwater or steam in which case additional control algorithms that vary thesteam flow in desirable ways are possible.

Coupled Approaches:

-   -   1 Fixed Air Flow, vary Fuel Flow over range;    -   2 Fixed Fuel Flow, vary Air Flow over range; and    -   3 Alternate between 1 and 2 above at period t interval and        frequency f maintaining a constant control setpoint.

Decoupled Approach: (i) control Tmax with fuel flow, over a span, formajority of time in closed loop; (ii) periodically, when Tmin=T1, switchto closed loop control with air to control Tmax while fuel loop is openloop with constraint such that Tmax does not exceed Th-max for period oftime, t; and (iii) after period of time t, return to closed loop onfuel, and open loop on air.

Hybrid on Tmin rate of decline: If the rate of decline of Tmin, R, isequal to R1, where is R1 is small, then implement the Decoupled Approachprovided above; if rate of decline of Tmin is moderate to large, thenimplement Coupled Approach 3.

Although the invention has been described in considerable detail, itwill be apparent that the invention is capable of numerous modificationsand variations, apparent to those skilled in the art, without departingfrom the spirit and scope of the invention.

1. A method for controlling the operating temperature of a reactorcomprising: a) setting an initial setpoint temperature for reactoroperation; b) operating the reactor at the initial setpoint temperature;c) measuring a maximum observed operating temperature within thereactor; d) measuring a minimum observed operating temperature withinthe reactor; e) calculating a temperature differential between themaximum observed operating temperature and the minimum observedoperating temperature; f) modifying the initial setpoint temperaturebased upon the temperature differential; and g) operating the reactor atthe modified setpoint until the temperature differential is less than aselected threshold.
 2. The method of claim 1 wherein the selectedthreshold of step (g) is less than about 350° C.
 3. The method of claim1 wherein the selected threshold of step (g) is less than 150° C.
 4. Themethod of claim 1 wherein step (g) further comprises operating thereactor at an increased maximum observed operating temperature withinthe reactor until the temperature differential is less than about 150°C.
 5. The method of claim 1 wherein step (g) further comprises operatingthe reactor at an increased maximum observed operating temperaturewithin the reactor until the temperature differential is less than about350° C.
 6. A method for controlling the operating temperature of areactor comprising: a) setting an initial setpoint temperature forreactor operation; b) operating the reactor at the initial setpointtemperature; c) measuring a minimum observed operating temperaturewithin the reactor; d) measuring a maximum observed operatingtemperature within the reactor; e) modifying the initial setpointtemperature based upon the minimum observed operating temperature; andf) operating the reactor at the modified setpoint until the minimumobserved operating temperature is greater than a selected threshold. 7.The method of claim 6 wherein the reactor comprises a catalytic partialoxidation reactor and the selected threshold of step (f) is greater thanthe threshold temperature for coking.
 8. The method of claim 7 furthercomprises operating the reactor at an increased maximum observedoperating temperature within the reactor until the minimum observedoperating temperature is greater than the threshold temperature forcoking.
 9. The method of claim 7 further comprises operating the reactorat an increased maximum observed operating temperature within thereactor until the minimum observed operating temperature is greater thanabout 800° C.